200 SURGICAL APPLIED ANATOMY, rchap. xt 



of retaining the clavicle in position after it is re- 

 placed. 



Acromio-claviciilar joint. This articulation 

 is shallow, and the outlines of the two bones that 

 enter into its formation are such that no obstacle 

 is offered to the displacement of the clavicle from the 

 acromion. The joint, indeed, depends for its strength 

 almost entirely upon its ligaments. The plane of the 

 joint would be represented by aline drawn from above 

 downwards and inwards between the two bones. This 

 inclination of the joint surfaces serves to explain the 

 fact that the usual luxation of this part takes the form 

 of a displacement of the clavicle upwards on to the 

 acromion. The capsule that surrounds the joint is lax 

 and feeble, and it is partly from its comparative 

 thinness that effusion into this joint, when it is the 

 seat of disease, makes itself so soon visible. The 

 joint, however, depends mainly for its strength upon 

 the powerful conoid and trapezoid ligamenta 



As the movements permitted in this joint may 

 be impaired by accident or disease, it is well to 

 note the part the articulation takes in the move- 

 ments of the extremity. The scapula (and with it, 

 of course, the arm), as it glides forwai'ds and back- 

 wards upon the thorax, moves in the arc of a circle 

 whose centre is at the sterno-clavicular joint, and 

 whose radius is the clavicle. As the bone moves 

 forwards it is important, for reasons to be imme- 

 diately given, that the glenoid cavity should also be 

 directed obliquely forwards. This latter desirable 

 condition is brought about by means of the acromio- 

 clavicular joint. Without this joint the whole 

 scapula as it passed forwards with the outer end 

 of the clavicle would precisely follow the line oi 

 the circle above mentioned, and the glenoid cavity 

 would look in an increasingly inward direction. It 

 is essential that the surface of the glenoid cavity 



